This invention relates generally to injection molding and more particularly to a multi-cavity valve gated injection molding system in which each valve member extends through a bushing which is received in an opening extending through the manifold. The bushing has a melt duct with two smoothly curved arms to convey melt from the manifold to the adjoining nozzle.
It is well known that providing for smooth flow of pressurized melt is critical to successful molding of certain materials. Sharp bends, corners or dead spots in the melt passage results in unacceptable resident time for some portion of the melt being processed which can cause too much delay on colour changes and/or result in decomposition of some materials or pigments of some materials such as polyvinyl chloride and some polyesters or other high temperature crystaline materials. In most multi-cavity valve gated injection molding systems it is necessary for the melt flow passage to change direction by 90.degree. and to join the bore around the reciprocating valve member as it extends from the manifold to each nozzle. Furthermore, substantial seepage of the pressurized melt as the valve member reciprocates must be avoided.
These problems necessarily require fine tolerance machining to overcome and it is well known to facilitate this by providing a separate bushing seated in the nozzle as disclosed in the applicant's U.S. Pat. No. 4,026,518 which issued May 31, 1977. A similar arrangement for multi-cavity molding is shown in the applicant's U.S. Pat. No. 4,521,179 which issued June 4, 1985. The applicant's U.S. Pat. No. 4,433,969 which issued Feb. 28, 1984 then shows a multi-cavity arrangement in which the bushing is located between the manifold and the nozzle. More recently U.S. Pat. No. 4,705,473 to Schmidt which issued Nov. 10, 1987 provides a bushing in which the melt duct in the bushing splits into two smoothly curved arms which connect to opposite sides of the valve member bore. Finally, U.S. Pat. No. 4,740,151 to Schmidt, et al which issued Apr. 26, 1988 shows a multi-cavity system with a different sealing and retaining bushing having a flanged portion mounted between the manifold and the back plate.
Of course, the bushing must have a minimum thickness (or height) to provide the necessary melt passage and the seal around the reciprocating valve member. On the other hand, reducing the overall thickness (or height) of the system is often very important, particularly in view of the recent trend to system size reduction. Locating the bushing between the manifold and the nozzle has the disadvantage that it increases the system height and providing separate bushings on opposite sides of the manifold only magnifies the problem.